Linux Virtualization - Dec 2007 - [virtio-net][PATCH] Don't arm tx hrtimer with a constant 500us each transmit

commit 763769621d271d92204ed27552d75448587c1ac0
Author: Dor Laor <dor.laor@qumranet.com>
Date:   Wed Dec 12 14:52:00 2007 +0200

    [virtio-net][PATCH] Don't arm tx hrtimer with a constant 50us each 
transmit
   
    The current start_xmit sets 500us hrtimer to kick the host.
    The problem is that if another xmit happens before the timer was 
fired then
    the first xmit will have to wait additional 500us.
    This patch does not re-arm the timer if there is existing one.
    This will shorten the latency for tx.
   
    Signed-off-by: Dor Laor <dor.laor@qumranet.com>

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 7b051d5..8bb17d1
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -406,10 +405,10 @@ again:
         virtio_debug(vdebug, "%s: before calling kick %d\n", 
__FUNCTION__, __LINE__);
         vi->svq->vq_ops->kick(vi->svq);
         vi->out_num = 0;
-    } else {
-        vi->stats.hrtimer_starts++;
-        hrtimer_start(&vi->tx_timer, ktime_set(0,500000),
-                  HRTIMER_MODE_REL);
+    } else if (!hrtimer_is_queued(&vi->tx_timer)) {
+            vi->stats.hrtimer_starts++;
+            hrtimer_start(&vi->tx_timer, ktime_set(0,500000),
+                      HRTIMER_MODE_REL);
     }
     return 0;
 }

Am Mittwoch, 12. Dezember 2007 schrieb Dor Laor:
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -406,10 +405,10 @@ again:
Hmm, while I agree in general with the patch, I fail to find the proper 
version of virtio_net where this patch applies. I tried kvm.git and 
linux-2.6.git from kernel.org. Can you give me a pointer to the repository 
where you work on virtio?

Christian

On Wednesday 12 December 2007 23:54:00 Dor Laor wrote:
> commit 763769621d271d92204ed27552d75448587c1ac0
> Author: Dor Laor <dor.laor@qumranet.com>
> Date:   Wed Dec 12 14:52:00 2007 +0200
>
>     [virtio-net][PATCH] Don't arm tx hrtimer with a constant 50us each
> transmit
>
>     The current start_xmit sets 500us hrtimer to kick the host.
>     The problem is that if another xmit happens before the timer was
> fired then
>     the first xmit will have to wait additional 500us.
>     This patch does not re-arm the timer if there is existing one.
>     This will shorten the latency for tx.
Hi Dor!

    Yes, I pondered this when I wrote the code.  On the one hand, it's a 
low-probability pathological corner case, on the other, your patch reduces 
the number of timer reprograms in the normal case.

So I've applied it, thanks!
Rusty.




>
>     Signed-off-by: Dor Laor <dor.laor@qumranet.com>
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 7b051d5..8bb17d1
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -406,10 +405,10 @@ again:
>          virtio_debug(vdebug, "%s: before calling kick %d\n",
> __FUNCTION__, __LINE__);
>          vi->svq->vq_ops->kick(vi->svq);
>          vi->out_num = 0;
> -    } else {
> -        vi->stats.hrtimer_starts++;
> -        hrtimer_start(&vi->tx_timer, ktime_set(0,500000),
> -                  HRTIMER_MODE_REL);
> +    } else if (!hrtimer_is_queued(&vi->tx_timer)) {
> +            vi->stats.hrtimer_starts++;
> +            hrtimer_start(&vi->tx_timer, ktime_set(0,500000),
> +                      HRTIMER_MODE_REL);
>      }
>      return 0;
>  }